The subject matter described herein relates to fasteners. More particularly, the disclosure relates to a self expanding fastener, method of using a self-expanding fastener, and structural assemblies comprising self expanding fasteners.
Forming structures for vehicles such as aircraft, rockets, motor vehicles or ships presents a need to join two or more structural components. For example, an aircraft wing skin needs to be joined to a spar or rib, a fuselage skin to a frame or stringer/longeron or a skin to another skin section through a lap splice joint. Various types of fasteners may be used to join such components.
Fasteners referred to as Class I fasteners in the aerospace industry provide effective load transfer by shear through contact of the mechanical fastener with the structural components that make up the joint. Many aerospace structures utilize Class I fasteners which have diameters that measure approximately 0.003″ less than the hole size. A Class I fastener has a tight fit of the fastener to its hole, reduces the Hertzian contact stress in bearing with the hole sidewall, and does not create interference of the fastener with the hole facilitating fastener insertion.
Non interference fasteners do not completely fill the fastener hole, resulting in an unfilled hole which has a stress concentration at the edge of the hole, which in turn lowers the fatigue life of a structure. The stress concentration on a unfilled hole means that a structure made of an alloy such as, for example, a Ti-6Al-4V, a common titanium alloy, will begin to plastically deform at the hole edge at a stress level of only 40 Ksi compared to this alloy's normal yield stress of 120 Ksi which is the stress where yielding would begin to occur in a structure that does not have a hole.
Fasteners that have a shank which is larger than the size of the hole(s) into which the fastener is fit, these are commonly referred to as interference fit fasteners. Interference fit fasteners have an advantage over Class I fastener joints in that they create no stress raiser in the structural components being joined, yielding a long fatigue life. However, they are not often used because specialized freeze insertion is typically required to fit the fastener in the hole.
Thus, stronger, lighter joints with longer fatigue lives can be created with any fastener that can fill a fastener hole to remove the stress concentration effect of the hole. Further, the fastener should be easily inserted into the hole, and should leave compressive residual stresses on the hole surface to increase the fatigue life of the structure.